Lactone, such as .gamma.-butyrolactone, may be used as a herbicide, used in the pharmaceutical composition, or used for preparing an intermediate; this intermediate is used to prepare pyrrolidone (such as N-methylpyrrolidone, 2-pyrrolidone and N-vinylpyrrolidone), piperidine, phenylbutyric acid and thiobutyric acid. Thus, developing an economic method for preparing .gamma.-butyrolactone is a common industrial requirement.
Previously, .gamma.-butyrolactone was commonly produced by a hydrogenation reaction with maleic anhydride or maleic acid ester under liquid phase or gas phase. However, this is an undesirable industrial process because the process needs a lot of hydrogen gas and its catalyst has a short lifetime.
Recently, .gamma.-butyrolactone was produced by a dehydrocyclization reaction with 1,4-butanediol, where a hydrogen by-product was generated as a raw material and as fuel. This method for preparing .gamma.-butyrolactone through the dehydrocyclization reaction of 1,4-butanediol is disclosed in Japanese Patent Unexamined Publication No. Sho-58-13575, wherein the dehydrocyclization reaction is carried out under liquid phase using a platinum/lead catalyst; however, the activity of said catalyst is low and the selectivity of .gamma.-butyrolactone is also low. The method disclosed in Japanese Patent Unexamined Publication No. Sho-61-246173 describes that .gamma.-butyrolactone is obtained by passing 1,4-butanediol vapor through a copper/chromium/zinc catalyst; however, this method may generate many tetrahydrofuran and butanol by-products, and the selectivity and yield of .gamma.-butyrolactone is usually low. The method disclosed in Japanese Patent Unexamined Publication No. Hei-3-232874 describes that .gamma.-butyrolactone is produced by passing 1,4-butanediol vapor through a copper/chromium/manganese or barium catalyst; the method disclosed in U.S. Pat. No. 5,110,954 describes that .gamma.-butyrolactone is obtained by adding 1,4-butanediol into the solution of copper/chromium catalyst; and the method disclosed in Japanese Patent Unexamined Publication No. Hei-2-255668 describes that .gamma.-butyrolactone is produced by passing 1,4-butanediol vapor through a copper/zinc/alkali metal catalyst. However, the activity of these catalysts may decay quickly, and the conversion of 1,4-butylene glycol may become lower after reacting over a long period. Therefore, it is undesirable as an industrial process.
The present inventors have deeply studied the above defects of the traditional technique and found more effective catalyst for preparing a lactone in the catalyst supporting a cupric compound, a zinc compound and at least one alkaline earth metal compound. The process for preparing lactone through the dehydrocyclization reaction of diol using the above catalyst under gas phase may increase the activity and lifetime of catalyst, while the selectivity may be up to 99 mol % or more; therefore the beneficial economic effect of industrial processes may be substantially increased. We have hereby accomplished the present invention.